This invention relates to coatings for infra-red transparent materials and applies particularly, but not exclusively, to coatings for multispectral window materials.
Infra-red (IR) radiation means radiation which is in the wavelength range in the region of approximately 1 .mu.m to approximately 14 .mu.m This includes a first IR radiation range of 3 .mu.m to 5 .mu.m and a second IR radiation range of 8 to 14 .mu.m. Radiation in the range 5 .mu.m to 8 .mu.m is largely absorbed by the atmosphere. Measured IR radiation can be used to determine a value of temperature of a body or to observe objects particularly when visible light levels are low.
IR detectors are usually relatively fragile and need to be protested from their environment, for example atmospheric conditions and physical contact. Therefore they are placed behind windows or domes which are made of materials transparent to the radiation that is being detected. Since many IR window materials are inherently soft and have poor scratch reistance, they suffer from erosion. Airborne systems suffer from rain, sand and insect damage inflicted at high velocities. In land based systems a combination of sand and window wipers can cause scratches.
It is known to protect IR windows by coating them with a hard protective coating. Known protective coatings for IR windows include diamond-like carbon (which is disclosed in British Patent GB 2082562 B) and boron phosphide (which is disclosed in British Patent GB 2223510 B).
However, a disadvantage of the use of such coatings is that they do not transmit radiation at visible wavelengths. The term visible refers to wavelengths approximately between 300 nm and 1000 nm (1 .mu.m). Since sensor systems may comprise detectors to detect visible radiation in addition to IR and it is convenient to locate both kinds of detector behind the same window, a hard multi-wavelength coating material is needed.